An airflow check would be done when a kiln is commisioned to verify that the airflow is what was ordered and to make sure it is uniform throughout the kiln.

Later, an airflow check might be done when MC variability occurs.  Although, if I suspected airflow problems I would first check the fan system to make sure they are all pushing air and doing it the correct direction.  And check baffling too. 

I guess there is no recommended interval.  Checking air flow and airflow distribution after kiln modification or changes in stacking practices makes sense.  For example, if you added coil baffles or widened your loads, these might be a reasons to check airflow.

Mike M.